Sense resistor short detection for switching power converters

Abstract

Systems, devices, and methods are provided to enable power converters to use two or more sensing elements to reliably detect a short circuit (or soft-short circuit) existing in the power converter and distinguish these conditions from other operating conditions, such as low voltage conditions or normal operating conditions. The disclosed embodiments enable the accurate detection of fault conditions, such as short circuits or soft short conditions, using a number of sensing elements, such as sense resistors and the inherent resistance of switching devices, while preventing the detection of false positives due to other factors such as low input voltage.

We claim: 
     
       1. A controller for a power switch transistor in a switching power converter comprising: a first comparator for comparing a sense resistor voltage across a sense resistor in series with the power switch transistor to a first threshold voltage to form a first comparator output signal;
 a second comparator for comparing a power switch voltage across the power switch transistor to a second threshold voltage to form a second comparator output signal; 
 a maximum on-time timer configured to time a maximum on-time period for the power switch transistor; and 
 a controller including a logic circuit configured to detect a short circuit condition for the sense resistor responsive to the first comparator output signal indicating that the sense resistor voltage is less than the first threshold voltage at an expiration of the maximum on-time period and the second comparator output signal indicating that the power switch voltage is greater than the second threshold voltage at the expiration of the maximum on-time period, wherein the logic circuit is further configured to detect a low input voltage condition for the switching power converter responsive to the first comparator output signal indicating that the sense resistor voltage is less than the first threshold voltage at the expiration of the expiration of the maximum on-time period and the second comparator output signal indicating that the power switch voltage is less than the second threshold voltage at the expiration of the maximum on-time period, and wherein the controller is further configured to shut down operation of the switching power converter in response to a detection of the short condition for the sense resistor. 
 
     
     
       2. The controller of  claim 1 , wherein the first comparator is configured to assert the first comparator output signal to a binary one state responsive to the sense resistor voltage being less than the first threshold voltage. 
     
     
       3. The controller of  claim 1 , wherein the second comparator is configured to assert the second comparator output signal to a binary one state responsive to the power switch voltage being greater than the second threshold voltage. 
     
     
       4. The controller of  claim 1 , wherein the controller is a primary-side controller for a flyback converter. 
     
     
       5. The controller of  claim 1 , wherein the power switch transistor is an NMOS power switch transistor, and wherein the second comparator is configured to receive the power switch voltage as a voltage across the NMOS power switch transistor. 
     
     
       6. The controller of  claim 1 , wherein the controller is further configured to cycle a cascode power switch in series with the power switch transistor. 
     
     
       7. The controller of  claim 1 , wherein the second threshold voltage equals a product of an on-resistance of the power switch transistor and a peak current of the power switch transistor. 
     
     
       8. The controller of  claim 1 , wherein the first threshold voltage equals a product of a resistance of the sense resistor and a peak current of the power switch transistor. 
     
     
       9. A method of detecting a sense resistor short condition and a low input voltage condition in a switching power converter, the method comprising:
 comparing a sense resistor voltage across a sense resistor to a first threshold voltage to form a first comparison signal; 
 comparing a power switch voltage across a power switch transistor in series with the sense resistor to a second threshold voltage to form a second comparison signal; 
 timing a maximum on-time period for the power switch transistor; 
 detecting the sense resistor short condition responsive to both the first comparison signal indicating that the sense resistor voltage is less than the first threshold voltage at an expiration of the maximum on-time period and the second comparison signal indicating that the power switch voltage is greater than the second threshold voltage at the expiration of the maximum on-time period; 
 detecting the low input voltage condition responsive to both the first comparison signal indicating that the sense resistor voltage is less than the first threshold voltage at the expiration of the maximum on-time period and the second comparison signal indicating that the power switch voltage is less than the second threshold voltage at the expiration of the maximum on-time period; and 
 shutting down operation of the switching power converter in response to the detection of the sense resistor short condition. 
 
     
     
       10. The method of  claim 9 , wherein comparing the power switch voltage comprises comparing a voltage across an NMOS power switch transistor.